In a printed matter required to have water resistance, like an outdoor poster or a label attached to a container for frozen food, a thermoplastic resin film such as polyethylene, polypropylene and polyethylene terephthalate film is used. Particularly, a white polyolefin-based synthetic paper with improved suitability for printing ensures that gravure printing, flexographic printing, letter press printing, offset printing or the like can be performed and an colorful printed matter can be obtained.
Such a polyolefin-based synthetic paper is usually subjected to an appropriate surface treatment to impart suitability for printing, such as ink transferability and ink adhesion, because the polyolefin-based resin used as a raw material has no polarity. As one of these surface treatments, a method of applying an oxidation treatment to the resin film surface and then coating a coating agent is known. As for the oxidation treatment, a method such as corona discharge treatment and flame treatment is known.
With respect to the coating agent, for example, a technique of coating a polyethyleneimine or an ethylene-imine.ethylene urea copolymer as the coating agent is disclosed in JP-A-01-141736 (the term “JP-A” as used herein means an “unexamined published Japanese patent application”), JP-A-06-025447 and JP-A-2000-29041.
However, the surface of such a surface-treated polyolefin-base synthetic paper is readily scratchable and when the printed matter is scratched in a processing step such as cutting and bookbinding, the ink is separated from the surface to cause a problem in practice. In the case where preventing this ink separation (scratch resistance) is necessary, it is required to use an abrasion-resistant ink at the printing or apply a surface varnish processing during or after printing and the cost becomes very high as compared with a normal printed matter.
An object of the present invention is to provide a thermoplastic resin film ensuring ink adhesion in a printed matter obtained by gravure printing, flexographic printing, letter press printing, offset printing or the like and moreover, imparting scratch resistance to the printed matter.